Recent years have seen widespread use of hybrid cars that use an engine and an electric motor in combination, as well as increasing use of electric cars. Such vehicles that use an electric motor as a power source are equipped with a high-voltage power source that outputs a high voltage of, for example, about 288 V to 600 V as the power source for driving the motor. Such a high-voltage power source is formed by a battery pack of a plurality of series-connected rechargeable batteries such as, for example, lithium ion rechargeable batteries or nickel metal hydride rechargeable batteries.
A high-voltage circuit, which is a circuit connected to the high-voltage power source, including the motor or an inverter to which power supply voltage is supplied from this high-voltage power source, or wires and the like for distributing the power supply voltage to the motor and the like, is insulated from the vehicle body. This structure thereby prevents a user from getting electric shocks when the user touches the vehicle body.
A lead storage battery of, for example, 12 V, is mounted in vehicles as a low-voltage power source for supplying power supply voltage to equipments that operate at a low voltage such as electrical equipments including in-vehicle stereos, lights, car navigation systems, or ECUs (Electronic Control Units), or the like. The body of the vehicle forms a circuit ground of a low-voltage circuit of electrical equipment or ECUs to which power supply voltage is supplied from this low-voltage power source. Namely, the vehicle body constitutes a circuit ground that is part of the low-voltage circuit, which is insulated from the high-voltage circuit.
Vehicles equipped with such high-voltage parts use an insulation resistance detection circuit that measures insulation resistance between the high-voltage circuit and the vehicle body (low-voltage circuit). For example, in the event of a ground fault due to, for example, a cable with damaged coating contacting the vehicle body, causing the insulation resistance between the high-voltage circuit and the vehicle body to drop, such a drop in the insulation resistance is detected by the insulation resistance detection circuit, and an alarm light in the in-vehicle instrument panel is turned on to draw attention from a passenger or a service man.
As such an insulation resistance detection circuit, a technique for detecting insulation resistance is conventionally known, wherein an alternating signal for measurement is output to a high-voltage circuit via a resistor and a capacitor (hereinafter referred to as a coupling capacitor), and a voltage appearing at a connection point between a resistor and the coupling capacitor is detected by an A/D converter, to detect insulation resistance from the signal amplitude of the detected voltage (see, for example, Patent Document 1). Since such an insulation resistance detection circuit itself is a low-voltage circuit, it is connected to the high-voltage circuit via the coupling capacitor to shut off DC current so as to maintain insulation between the high-voltage circuit and the low-voltage circuit.
In such an insulation resistance detection circuit, the signal amplitude mentioned above changes in accordance with a voltage divider ratio determined by the series impedance of the coupling capacitor and the insulation resistance, and the above-mentioned resistor, so that insulation resistance can be detected from the signal amplitude.
However, in the insulation resistance detection circuit described above, the moment a ground fault occurs, the voltage (potential) on the side of the insulation resistance detection circuit of the coupling capacitor undergoes an instantaneous change because of the high voltage of the high-voltage power source, whereby DC potential input to the A/D converter changes by several hundreds volts. Since the input voltage range of an A/D converter is typically about 5 V to 10 V, the voltage of the coupling capacitor falls out of the input voltage range of the A/D converter, as a result of which the above-mentioned signal amplitude becomes undetectable.
After that, the above-mentioned signal amplitude cannot be detected by the A/D converter until after the coupling capacitor has been charged and discharged by the alternating signal for measurement in accordance with a time constant thereof so that the voltage of the coupling capacitor has fallen back into the input voltage range of the A/D converter, and therefore, insulation resistance cannot be detected, either. This means there is a time lag between a drop in insulation resistance caused by a ground fault or the like and issuance of an alarm to a passenger or a service man, which is not desirable.
Since this time lag is caused by the time constant associated with charging and discharging of the coupling capacitor, reducing the capacitance of the coupling capacitor is assumed to shorten the time lag from occurrence of a ground fault until the insulation resistance becomes detectable.
However, the problem was that reducing the capacitance of the coupling capacitor would cause the voltage thereof more variable because of the influence of noise during driving of the vehicle or inverter noise and the like, which would lead to erroneous detection of insulation resistance and malfunctions.    Patent Document 1: Japanese Patent Application Laid-open No. 2004-104923